The same requirements exist for the insulation of underground electrical power distribution and transmission cables as for overhead power transmitting or distribution systems. However, certain additional factors must be taken into consideration when an underground system is considered in lieu of an overhead layout. Two of the principal considerations are the increased hostility of the environment and the relative inaccessibility of the cable once it is placed in the ground.
Underground cables are constantly exposed to moisture which is in the ground, which when coupled with the voltage stress generated by the electrical field, can cause electrochemical treeing (water trees) in the cable insulation. This treeing effect will eventually destroy the insulation to the point where it is ineffectual and the cable is no longer fit for use. In an attempt to combat the corona effect on the insulation, the cable industry has resorted to the addition of a highly conductive layer which acts as a ground electrostatic shield and will drain the extraneous electrical field away from the cable. Additionally, a number of sheathings have been designed to be moisture barriers and used in wraps around the cable, U.S. Pat. Nos. 4,256,921 and 4,145,567. These sheaths, because they are an integral component of the cable, are subject to thermal expansion when the cable is in use. This expansion can cause fatigue failures in the sheath reducing its effectiveness. FIG. 1 is a typical approach which the cable industry has taken to solve the problem. Such cables 1 comprise an electrically conductive core 2 surrounded by a layer of semiconductive polymeric material 3, an electrically insulating layer 4, an outer semiconductive layer 4a, and a moisture resistant and abrasion resistant layer 5 having spirally embedded within it, rods 6 of conductive material which act as a concentric neutral or return conductor for single phase application. Typically copper is used to form the concentric neutral due to its resistance to corrosion. However, copper is relatively expensive and since the concentric neutral constitutes a major cost of the cable itself, adds greatly to the cost of the cable.
In addition to the moisture problem, underground cables are exposed to rough and jagged rocks which can abrade or damage the cable insulation, again making them unfit for service. Due to these and other problems, present in-ground electrical cables do not last forever; in general, their useful life is only about ten to fifteen years.
These electrical power cables, whether used aboveground or underground, are very expensive. Therefore, when a portion of a cable network fails, it would be desirable to be able to locate that portion and repair it by splicing or another technique. This is easily accomplished when the cable network is an overhead system, but not so easy when the cable is buried underground.
When the cable is buried underground, and it fails, it may be possible to locate that portion which is faulty, but repairing it may be much more difficult. In many instances, it may be virtually impossible to unearth the faulty cable due to construction activities which have taken place above the cable subsequent to the cable being interred. For example, buildings, landscaping, roads or airport runways may have been built in the interim, making the cost of replacement economically prohibitive as well as physically difficult. The alternative, then, is to lay down an entirely new cable.
Therefore, what is needed in this art is an improved underground cable and conduit system capable of protecting the underground cable from moisture and abrasion, while allowing the cable to be relatively easily replaced.